The present invention generally relates to radio-frequency (RF) communication networks, and, more particularly, to a system for alignment of transmission times of multiple RF signals transmitted by a user equipment to an RF transceiver.
A RF communication network includes a plurality of RF communication systems, such as base transceiver stations (BTSs) and user equipments (UEs). The BTS and UEs communicate using RF signals. The RF communication network may conform to specific standards and technologies like long term evolution (LTE), LTE advanced (LTE-A), high speed packet access (HSPA), and other third generation partnership project (3GPP) standards. In LTE, the BTS is referred to as an eNode-B (or eNB). The eNB includes a RF transceiver for transmitting and receiving the RF signals to and from the UEs. The RF transceiver modulates a carrier wave by changing at least one of the characteristics of the carrier wave, viz. amplitude, frequency and phase based on a digital baseband signal and then transmits the digital baseband signal using the modulated carrier wave on a transmission medium using an antenna.
The eNB communicates with multiple UEs simultaneously by transmitting RF signals on multiple RF sub-carrier frequencies that are modulated over an operating carrier frequency. The eNB uses Orthogonal Frequency-Division Multiple Access (OFDMA) to distinguish between the RF signals received from the UEs. In OFDMA, each UE is assigned a set of RF sub-carriers and a set of sub-frames for transmitting the RF signals to the eNB on the assigned set of RF sub-carriers. The eNB further assigns a time offset value to each UE that is indicative of a specific time at which the RF signals transmitted by the UE (hereinafter referred to as uplink RF signals) are expected to be received at the eNB. The UE transmits the RF signals in the assigned set of sub-frames on the assigned set of RF sub-carriers.
However, in an LTE cell, the UEs can be located at varying distances from the eNB. The RF signals transmitted by the UE to the eNB are subject to an uplink propagation delay based on the distance of the UE from the eNB. Therefore, the propagation delays for the uplink RF signals transmitted by the UEs differ based on the distance of each UE from the eNB. Similarly, the RF signals transmitted by the eNB to the UE are subjected to a downlink propagation delay. The uplink and downlink propagation delays are collectively termed as a round-trip propagation delay. To align the uplink RF signals transmitted by different UEs with the specific time at the eNB, the round-trip propagation delay must be compensated for such that the uplink RF signals from the UEs arrive at the specified time at the eNB. When the uplink RF signals from a first UE arrive at a time other than their specified time, then the RF signals will arrive at the eNB in the set of sub-frames assigned to another UE, which may result in a loss of information transmitted by the UEs.
The UEs transmit reference signals to the eNB such as a sounding reference signal (SRS) and a demodulation reference signal (DMRS). The eNB calculates a specific time alignment value for each UE based on the timing of these reference signals. The time alignment value is a time offset between transmission time of the uplink RF signals and a desired transmission time of the uplink RF signals. Thus, the time alignment values are indicative of a change in a transmission time offset of the uplink RF signals that is required for aligning the uplink RF signals with a predetermined uplink time offset computed by the eNB. The eNB transmits a time alignment command that includes the time alignment value to the UE in downlink sub-frames to compensate for the round-trip propagation delay.
The UE adjusts the time offset of the uplink RF signals based on the received time alignment value after a predetermined count of sub-frames. In an LTE-based RF communication network, the count of sub-frames after which the UE adjusts the time offset of the uplink RF signals is determined by the LTE specification standard. For example, the UE may adjust the time offset of the uplink RF signals after receiving six sub-frames subsequent to reception of the time alignment value from the eNB. However, there is a possibility that the UE will receive additional time alignment values in the six sub-frames that the UE receives after receiving the time alignment value. For example, the UE may accumulate the time alignment value and the additional time alignment values to determine a cumulative time alignment value. Further, after receiving six sub-frames after receiving the time alignment value from the eNB, the UE may determine a time period corresponding to the cumulative time alignment value and adjust the time offset of the uplink RF signals based on the determined time period.
For example, when the cumulative time alignment value exceeds a default time alignment value, the UE will advance the uplink RF signals by the time period corresponding to the cumulative time alignment value. The default time alignment value is the time alignment value that indicates that the uplink RF signals are aligned to the predetermined uplink time offset transmitted by the eNB and that the adjustment to the transmission time of the uplink RF signals is not required. Similarly, when the cumulative time alignment value is less than the default time alignment value, the UE will delay the uplink RF signals by the time period corresponding to the cumulative time alignment value. For example, for the LTE-based RF communication network, the default time alignment value is thirty-one. Hence, for the cumulative time alignment value of thirty-two, the UE advances the uplink RF signals by a time period corresponding to “1TA” and for the cumulative time alignment value of thirty, the UE delays the uplink RF signals by a time period corresponding to “1TA”. In one example, 1TA is 0.5208 microseconds.
Referring to FIG. 1A, a timing diagram illustrating a plurality of sub-frames of an RF signal received by a UE from a conventional eNB is shown. The UE receives a plurality of sub-frames 102a-102f from the eNB. The sub-frames 102a-102d received during a time period N0-N4 include time alignment (TA) commands 104a-104d. Each TA command of the TA commands 104a-104d is indicative of a TA value of thirty-two. The TA value of thirty-two corresponds to 1TA. The sub-frames 102e and 102f received during the time period N4-N6 do not include any TA command.
Referring now to FIG. 1B, a graph illustrating an accumulated TA value corresponding to the TA commands 104a-104d received by the UE in the sub-frames 102a-102d, respectively, is shown. Initially, the UE receives the TA command 104a in the sub-frame 102a. The TA value corresponding to the TA command 104a is thirty-two, which indicates that the UE requires advancement of the transmission time of the uplink RF signals by 1TA for aligning the uplink RF signals with the predetermined uplink time offset computed by the eNB. However, as per the LTE standard, the UE adjusts the time offset of the uplink RF signals at time N6 after receiving the TA command 104a. Since the UE does not adjust the time offset of the uplink RF signals before time N6, the time offset of the uplink RF signals is unchanged for the time period NO-N4, and hence, the eNB transmits the TA commands 104b-104d indicative of the time advance of 1TA each in the transmission time of the uplink RF signals. Therefore, the UE accumulates the TA commands 104a-104d received during the time period NO-N6 to determine a cumulative TA value. The cumulative TA value equals a sum of the accumulated TA commands, i.e., the cumulative value is 4TA. The cumulative TA value of 4TA indicates that the uplink RF signals require time advancement of 4TA. Hence, at time N6, the UE advances the uplink RF signals by 4TA. Therefore, the UE advances the transmission time of the uplink RF signals by 4TA instead of advancing the transmission time of the uplink RF signals by 1TA as indicated initially by the TA command 104a, resulting in over compensation of the time offset of the uplink RF signals and misalignment of the uplink RF signals received at the eNB. Further, to compensate for the misalignment of the RF signals at the eNB, the eNB sends successive TA commands (not shown) in subsequent sub-frames (not shown) to the UE indicative of a delay in the transmission time of the uplink RF signals for aligning the time offset of the uplink RF signals with the predetermined uplink time offset computed by the eNB. Hence, the UE enters a state of oscillation between misaligned start positions of sub-frames of the RF signals, and therefore, loses synchronization with the eNB.
Referring now to FIG. 2A, a timing diagram illustrating a plurality of sub-frames of an RF signal received by another UE from another conventional eNB is shown. The UE receives a plurality of sub-frames 202a-202f from the eNB. The sub-frames 202b and 202c include TA commands 204a and 204b, respectively. The TA command 204a is indicative of a TA value of thirty-four and the TA command 204b is indicative of the TA value of thirty-two. The TA value of thirty-four corresponds to 3TA and the TA value of thirty-two corresponds to 1TA.
FIG. 2B is a graph illustrating an accumulated TA value corresponding to the TA commands 204a and 204b received by the UE in the sub-frames 202b and 202c, respectively. Initially, the UE receives the TA command 204a in the sub-frame 202b. The TA value corresponding to the TA command 204a is thirty-four, thereby indicating that the UE requires advancement of the transmission time of the uplink RF signals by 3TA for aligning the uplink RF signals with the predetermined uplink time offset computed by the eNB. However, the UE adjusts the time offset of the uplink RF signals at time N6 after receiving the TA command 204a. Hence, the eNB further transmits the TA command 204b indicative of further time advance of 1TA in the transmission time of the uplink RF signals. The UE accumulates the TA commands 204a and 204b received during the time period NO-N6 to determine a cumulative TA value of 4TA. At time N6, the UE advances the transmission time of the uplink RF signals by 4TA, thereby resulting in a loss of synchronization with the eNB.
One known way to overcome this loss of synchronization problem requires calculation of a first estimate of the time offset of the uplink RF signals by the eNB based on a reference RF signal transmitted by the UE. The eNB further calculates multiple estimates of the time offset based on the reference RF signals transmitted by the UE. The UE accumulates the received estimates of the time offset and generates a cumulative time offset estimate. The UE compares the cumulative time offset estimate with a predetermined threshold value. If the cumulative time offset estimate exceeds the threshold value, then the UE determines that synchronization with the eNB is lost and initiates a coarse synchronization procedure. During the coarse synchronization procedure, the UE searches for primary and secondary synchronization signals transmitted by the eNB. The threshold value is calculated by the UE based on a cyclic prefix of the RF signals. Hence, the technique does not provide any facility for an operator to modify the threshold value. Further, the technique does not provide any way to avoid the loss of synchronization.
Another known technique to overcome the loss of synchronization problem includes the use of a time alignment timer in the UE. The UE initializes the time alignment timer when the UE receives a TA command from the eNB. The timer runs for a preset time period and then expires. When the timer expires, the UE initiates a random access procedure to obtain uplink synchronization. During the random access procedure, the UE transmits a random access preamble to the eNB. Further, the UE reinitializes the time alignment timer after receiving another TA command after the random access procedure. A finite time period is required for performing the random access procedure. Since this technique requires the UE to perform the random access procedure multiple times, a large amount of time is consumed in performing the random access procedure, thereby reducing the efficiency of the UE.
In yet another known technique to overcome the aforementioned problem in a global system of mobile communication (GSM) based RF communication network, the UE receives and stores the TA command transmitted by a base station (hereinafter referred to as “BTS”) during an initial synchronization procedure. The UE detects a relative movement of the UE with respect to the BTS to determine whether the UE is stationary mode or in motion. The technique uses a global positioning system (GPS) unit for the detection of the relative movement. Alternatively, an accelerometer or measurement of the timing of the RF signals transmitted by the BTS can be used to determine the relative movement of the UE with respect to the BTS. When the UE is stationary, the UE transmits the uplink RF signals with the time offset indicated by the stored TA command. When the UE is in motion, it initiates a random access procedure to receive another TA command from the BTS. However, because the UE is configured as a default to operate in the stationary mode, it changes back to the moving mode only when a specified number of attempts to transmit the uplink RF signals have failed. Hence, the technique requires a specified number of failed attempts before re-initiating the random access procedure, which reduces the performance of the UE.
Therefore, it would be advantageous to have a more efficient system for time alignment of RF signals in an RF communication network.